Component with transparent background to BufferedImage?

Question

I have a component extends JPanel. It saves itself as bufferedimage on every call of paintComponent method. Component is not completely transparent, only its background. Problem is background is not transparent. I am using setOpaque(false);

Here is my relevant code;

private BufferedImage bufImage = null;

public void paintComponent(Graphics g) {
    super.paintComponent(g);
    Graphics2D g2 = (Graphics2D)g;

    // if first time call
    if (bufImage == null) {
        int w = this.getWidth();
        int h = this.getHeight();
        bufImage = (BufferedImage)this.createImage(w, h);
    }

    g2.drawImage(bufImage, null, 0, 0);

    // draw sth
    g2.draw(sth);
}

--

I also tried

bufImage =  new BufferedImage(w, h, BufferedImage.TYPE_INT_ARGB);

instead of

bufImage = (BufferedImage)this.createImage(w, h);

When i do that; background transperancy works but i can only draw with white color. I have no idea what causes that.

Note: I used that code to check if it is working;

File outputfile = new File("saved.png");
ImageIO.write(bufImage, "png", outputfile);

saved.png had transparent background but drawings were only white.

---

This is the component, only lets drawing rectangle with mouse;

class PaintPanel extends JPanel implements MouseListener, MouseMotionListener {
    private BufferedImage _bufImage = null;
    private boolean dragging = false;
    private Point _start = null, _end = null;

    public PaintPanel() {
        setOpaque(false);
        this.addMouseListener(this);
        this.addMouseMotionListener(this);
    }

    @Override
    public void paintComponent(Graphics g) {
        super.paintComponent(g);
        Graphics2D g2 = (Graphics2D)g;

        if (_bufImage == null) {
            int w = this.getWidth();
            int h = this.getHeight();
            _bufImage =  new BufferedImage(w, h, BufferedImage.TYPE_INT_ARGB);
            //_bufImage = (BufferedImage)this.createImage(w, h);
        }

        g2.drawImage(_bufImage, null, 0, 0);

        if (dragging) {
            drawCurrentShape(g2);
        }
    }

    private void drawCurrentShape(Graphics2D g2) {
        int startx = (int) _start.getX();
        int starty = (int) _start.getY();
        int stopx = (int) _end.getX();
        int stopy = (int) _end.getY();

        int width = Math.abs(startx - stopx);
        int height = Math.abs(starty - stopy);
        int x = startx, y = starty;
        if(x > stopx)
            x = stopx;
        if(y > stopy)
            y = stopy;

        Rectangle r = new Rectangle(x, y, width, height);
        g2.draw(r);
    }

    public void mousePressed(MouseEvent e) {
        dragging = true;       
        _start = e.getPoint();
        _end   = _start;
    }

    public void mouseDragged(MouseEvent e) {
        _end = e.getPoint();
        this.repaint();
    }

    public void mouseReleased(MouseEvent e) {
        _end = e.getPoint();
        if (dragging) {
            dragging = false;
            drawCurrentShape(_bufImage.createGraphics());  
            this.repaint();
        }
    }

    public void mouseMoved  (MouseEvent e) {}
    public void mouseEntered(MouseEvent e) {}
    public void mouseExited (MouseEvent e) {}
    public void mouseClicked(MouseEvent e) {}
}

Answer 1

try this:

bufImage = new BufferedImage(w,h,java.awt.image.BufferedImage.TYPE_INT_ARGB);
Graphics2D graphics = bufImage.createGraphics();
this.print(graphics);
graphics.dispose();

The key is to use print()

Edit: I tried the following and transparency works like a charm:

import java.awt.Dimension;
import java.awt.Graphics2D;
import java.awt.image.BufferedImage;
import java.io.File;
import java.io.IOException;

import javax.imageio.ImageIO;
import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.SwingUtilities;

public class Test2 {
    public static void main(String[] args) {
        JFrame frame = new JFrame();
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        final JPanel p = new JPanel();
        p.setPreferredSize(new Dimension(400, 400));
        p.setOpaque(false);
        JButton button = new JButton("Hello world");
        p.add(button);
        frame.add(p);
        frame.pack();
        frame.setVisible(true);
        SwingUtilities.invokeLater(new Runnable() {

            @Override
            public void run() {
                BufferedImage bufImage = new BufferedImage(p.getWidth(), p.getHeight(), java.awt.image.BufferedImage.TYPE_INT_ARGB);
                Graphics2D graphics = bufImage.createGraphics();
                p.print(graphics);
                graphics.dispose();
                try {
                    ImageIO.write(bufImage, "png", new File("d:/tmp/tmp.png"));
                } catch (IOException e) {
                    // TODO Auto-generated catch block
                    e.printStackTrace();
                }
            }
        });
    }
}

Answer 2

createImage(w, h) will create a "blank" image with the specified width and height. That being said, what you'll need to do is invoke createGraphics on the BufferedImage instance and draw directly to the returned Graphics object.